Project summary Remote and local connectivity of the prefrontal cortex is altered in mental disease, but little is known about underlying synaptic mechanisms. Emotional trauma is an environmental factor that increases odds of developing mental disease in the future, upon exposure to another traumatic event. We have recently shown that a rodent analog of emotional trauma, the observational fear paradigm (OF), in which mice are exposed to a conspecific in distress, enables stronger passive avoidance learning at a later time. Our preliminary analysis of synaptic transmission in the dorsomedial prefrontal cortex (dmPFC) of traumatized mice, which is essential for passive avoidance learning, revealed a redistribution of inhibition along the somato-dendritic axis of the layer V pyramidal cells (PCs) via a GABAb receptor-mediated mechanism. The objectives of this proposal are two-fold: first, to determine synaptic and cellular mechanisms of that redistribution by testing a hypothesis that OF uniquely modulates GABAb receptor signaling in genetically-distinct classes of GABAergic neurons, and, second, to examine its consequences for synaptic transmission between the basolateral amygdala (BLA) and dmPFC. Our pilot experiments showed that the two ascending pathways from BLA to dmPFC that arrive in layers II/III and V, can be optogenetically stimulated with high selectively using focal blue light and that such stimulation elicits different patterns of feedforward inhibition in the layer V PCs. A prediction will be tested that OF has opposing effects on inhibition recruited by these pathways. The proposal employs optogenetic stimulation of specific axonal fibers using localized focused light, pharmacogenetic silencing of specific interneuronal populations and recording of synaptic responses in dmPFC slices prepared from animals subjected to OF. The following questions will be addressed. Which types of GABAergic neurons are responsible for the somato-dendritic shift in inhibition? What is the subcellular compartment where GABAb receptor signaling is altered? Does OF alter the balance between excitation and feedforward inhibition elicited in layer 5 PCs by the two BLA inputs? Which interneurons mediate feedforward inhibition recruited by each input? Do glutamatergic inputs in those interneurons play a role in the effects of OF? By identifying cells, synapses and molecules that redistribute inhibition of L5 PC along somato- dendritic axis and by examining resulting changes in transmission via the BLA-dmPFC ascending pathway, this project will help determine mechanisms of the enhanced fear learning after psychological trauma and identify potential targets for novel therapies in PTSD, depression and related emotional disorders.